1. Diets enriched with n-3 polyunsaturated fish oils, predominantly eicosapentaenoic acid, are associated with a lower risk of atherosclerotic vascular disease. These oils purportedly reduce plasma triglycerides, total cholesterol and impair platelet aggregation. Recently, the present authors reported that rats fed a marine oil-enriched diet had significantly reduced levels of lyso-PAF, the immediate precursor of platelet-activating factor (PAF). As PAF has potent vasodilator and pro-aggregatory properties, the purpose of this study was to examine the hypothesis that fish oils affect the biosynthesis of PAF in man. 2. Supplementation of a normal diet for 3 weeks with fish oil containing the equivalent of 2.7 g of eicosapentaenoic acid daily, increased the eicosapentaenoic acid content of platelet phospholipids as well as depleting the arachidonic acid. Platelet aggregation to PAF (measured in whole blood by impedance aggregometry) was significantly impaired and whole blood thromboxane suppressed. 3. Two weeks after ceasing supplements, platelet aggregation remained impaired although thromboxane had reverted to baseline levels. There was a transient but significant fall in whole blood lyso-PAF apparent within 2 days of commencing supplements but returning to baseline levels by the end of the treatment period. Whole blood PAF followed a similar trend. 4. The effects of dietary fish oil on whole blood aggregations to PAF, on thromboxane and plasma lyso-PAF levels may be relevant to the prevention of vascular disease and the treatment of disorders in which PAF could be an inflammatory mediator.